The Difference between Heat Transfer Oils and Lubricating Oils

What is Heat Transfer Oil?

Heat Transfer Oil is a highly refined and stable paraffinic oil designed to be used as a heat transfer medium and quenching oil. In many industrial applications, heating is provided indirectly by circulating hot oil through a heat exchanger, thus reducing hot spots and increasing the safety of the heating process.What is Lubricating Oil?

Lubricating is an oily substance that is used to cover or treat machinery to lessen friction. Lubricating oils are specially formulated oils that reduce friction between moving parts and help maintain mechanical parts

The Difference between Heat Transfer Oil and Lubricating Oil

Heat Transfer Oil and Lubricating Oil are greasy, and they both transfer heat. But beyond this, they are made for different purposes. There is a difference between these two things that oil companies market. Certain terms and test results that one comes across in product data sheets pertain more to lube oils than heat transfer oils.

Viscosity Index – Viscosity Index (VI) is an arbitrary measure for the change of viscosity with variations in temperature. The lower the VI, the higher the difference of viscosity of the oil with temperature and vice versa. It is used to characterize viscosity changes with relation to temperature in lubricating oil. However, most lube oils are formulated for as little change in the viscosity as possible. For heat transfer, you want as much change as you can get.But is a high number better than a low number? And what is the viscosity at a given temperature?

SAE Grade / ISO Grade – Oil viscosity is measured in several ways. Two of the main ones that we come across in equipment manuals are SAE and ISO. SAE stands for Society of Automotive Engineers. Engine oil viscosity is always measured in terms of SAE. Next we have the ISO system which stands for International Standardization Organization. The ISO scale is commonly used to measure the viscosity of industrial oils.

SAE Grade / ISO Grade are general lubricating terms. The lower the number, the lower is the viscosity. These terms are normally used by a manufacturer for distinguishing between two product grades. They work for car oil but again provide no data that can be used for either selecting oil or designing systems.SUS (Saybolt Universal Seconds) – This is a viscosity unit that is only relevant to lubrication types of oil. Because the oil's viscosity changes with temperature, it is always expressed at a reference temperature, usually 100° and 210° F. The most common unit in the United States is the SUS which is measured with a Saybolt Viscosimeter. Heat Transfer oils use centistokes, centipoise or lbs/ft-hr for selecting pumps and sizing heat exchangers.

Copper Weathering – Copper is seldom used in heat transfer systems so corrosion is not an issue.

Resistance to Water/Non-foaming/Demulsibility – Only lube oils retain water long enough to cause system corrosion or foam. Take the case of a geyser of thermal fluid coming out of an expansion tank vent – the water doesn’t stay around a thermal fluid system for very long. It transforms to steam and forces an equal volume of liquid out of the system.

Conradsen Carbon Residue / Ramsbottom Carbon Residue / Carbon Residue (Micro Method) – These tests are used to measure carbon residues of oil. In brief, the carbon residue of fuel is the tendency to form carbon deposits under high-temperature conditions in an inert atmosphere. They determine how much solid material remains after a sample has been completely evaporated and then pyrolized at high temperatures while exposed to air. While theoretically an indication of the tendency of oil to form deposits, it is a measure of the number of inorganic additives since the pyrolysis step destroys most of the carbon in the residue.

Product literature has claims and properties that indicate that the product is not formulated only for heat transfer. Keep a watch out for this.

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